1. Field of the Invention
The present invention relates to superconducting cryogen level probes used to measure the level of liquid cryogen within a cryogen vessel.
2. Description of the Prior Art
Known cryogen level probes employ a length of superconductive wire, which is partially immersed, substantially vertically, into liquid cryogen contained within a cryogen vessel. Those parts of the superconductive wire in contact with liquid cryogen will be in the superconducting state, while those parts of the wire in gaseous cryogen may be in resistive state, as the gaseous cryogen is a much less effective coolant than the liquid cryogen.
FIG. 1 shows a cryostat such as may be employed for holding magnet coils for an MRI (magnetic resonance imaging) system. A cryogen vessel 1 holds a liquid cryogen 2. The space 3 in the cryogen vessel above the level of the liquid cryogen may be filled with evaporated cryogen. The cryogen vessel is contained in a vacuum jacket 4 which serves to reduce the amount of heat flowing to the cryogen 2 from ambient temperature, by reducing the possibility of conduction or convection heating of the cryogen vessel 1. One or more heat shields 5 may be provided in the vacuum space between the cryogenic vessel 1 and the vacuum jacket 4. These shields serve to reduce the amount of radiated heat reaching the cryogenic vessel 1 from the exterior. An access neck 6 is provided, allowing access to the cryogenic vessel from the exterior. This is used to fill the cryogenic vessel, to provide access for current leads and other connections to superconductive coils housed within the cryogenic vessel, and to allow an escape path for boiled-off gaseous cryogen.
In such systems, it is necessary to regularly monitor the level of the liquid cryogen, while the system is still in an operational state. This is necessary to detect leaks, indicated by an unusually high consumption of cryogen, and to ensure that the liquid cryogen is topped up at appropriate intervals so that the magnet coils or other articles remain sufficiently cooled by the liquid cryogen. At low cryogen levels, parts of the magnet will no longer be immersed in liquid cryogen and will be at a higher temperature than when the cryogen level is high. In the case of a superconducting magnet, this could lead to a magnet quench, which may be dangerous and damaging to the system, and cause the magnetic field to collapse. However, any selected measurement process should not represent an undue heat input to the system. It is generally regarded as sufficient to measure the level of liquid cryogen once per day.
A guide tube 10 is provided inside the cryogenic chamber for housing a cryogen level probe. The guide tube 10 runs from the access neck 6 to approximately the lower extremity of the cryogen vessel. The guide tube is not sealed at its end, but fills with liquid cryogen to the level of the liquid cryogen in the cryogen vessel. The guide tube 10 is provided to house a cryogen level probe for measuring the depth of the liquid cryogen 2 in the cryogenic vessel 1. The cryogen level probe comprises a superconductive wire running the length of the probe.
In operation, a current is passed through the wire. Those parts of the wire which are immersed in liquid cryogen will essentially remain in superconducting state, while those parts of the wire exposed to gaseous cryogen may become resistive. The voltage developed across the resistive part of the wire provides an indication of the resistance of the wire. This in turn indicates the length of the wire which is in the gaseous cryogen, and so provides an indication of the level of the liquid cryogen in the cryogen vessel.
Such cryogen level probes are known to have inconsistent operational performance in terms of reliably measuring the cryogen level in cryogenic vessels. A common application of cryogen level sensors is to measure the level of liquid helium in cryogen vessels containing superconducting magnets for MRI imaging systems. However, the present invention may be applied to the measurement of any cryogen, in any type of cryogen vessel.
Typically, a cryogen level probe includes a length of thin superconducting wire (typically 0.1 mm diameter) located within the guide tube 10, or otherwise within the cryogen vessel—for example, mounted on a non-conductive carrier or enclosed within a protective mesh surround.
In order to effect a measurement of cryogen level, an upper part of the superconducting wire, located in gaseous cryogen, is made resistive by heating with an electrical heater powered from an external electrical control device. Electrical current is passed through the superconducting wire, and heat dissipated in the resistive part of the superconducting wire heats adjacent parts of the wire, rendering those resistive. The electrical heater is wired in series with the superconductor, therefore is provided with the same current as the superconducting wire. In a known arrangement the electrical heater is a wire heater wrapped around the superconducting wire and bonded with varnish. In another known variant the superconducting wire lays over a foil heater, which is electrically in series with the superconducting wire. The result is that a resistive “front” propagates along the superconducting wire until the surface of the liquid cryogen is reached.
Conventionally, those parts of the wire in contact with the liquid cryogen are so efficiently cooled that the resistive front ceases to propagate once it reaches the surface of the liquid cryogen. The resistance of the superconducting wire, and so the voltage across it for a given applied current, at this time provides an indication of the level of the cryogen within the cryogen vessel.
A problem arises in balancing the introduction of sufficient energy to cause the resistive front to reach the liquid layer whilst preventing over-propagation of the resistive front below the surface of the liquid cryogen by the application of too much energy, to prevent a level below the liquid surface being measured. This issue is addressed in UK patent application GB2415512A, which describes measuring cryogen levels using a cryogen level probe, including a possible solution for “stuck” probes where propagation of a resistive front is interrupted by the presence of an unwanted heat loss path.
Cryogen vessels can have inhomogeneous temperature zones above the liquid level, for example particularly cold spots, which can occur unpredictably within a cryogenic vessel due to the complexity of the thermodynamic conditions within the vessel. If insufficient energy is applied to the superconducting wire of the probe, these cold zones will prevent the wire becoming resistive in those areas and therefore an incorrect level measurement will be obtained. Typically the critical temperature for the superconducting wire is 8 to 10 Kelvin for the expected magnetic field strengths in an MRI application and typical operating current. If, for example, liquid helium cryogen is used at a temperature of 4.2K, it is quite likely that regions of gaseous cryogen may be present at temperatures of 8-10K or less.
This invention ensures the reliable and accurate measurement of cryogen levels given the above conditions, overcoming known problems with the method described in GB2415512A.
Other conventional arrangements are described in the following documents. GB2401688A describes a construction of a Helium probe, but makes no reference to the configuration of current pulses applied to the probe in operation.
JP8035875-A1 describes a probe structure, and also mentions a detection circuit using an A-D converter.
JP61031925-A1 describes probe construction rather than control.
SU1272860-A1 describes a probe with a full length heater wound to it, with tapping points.